1. Field of the Invention
This invention relates to coated hard alloys or cemented carbides and more particularly, it is concerned with an alumina-coated tool having a particularly excellent cutting property.
2. Description of the Prior Art
At the present time, many coated hard alloys have been used as cutting tools excellent in wear resistance as well as in toughness in the field of mechanical working or machining, the coated hard alloys being produced by coating hard alloys consisting of at least one of carbides, nitrides, carbonitrides and carboxynitrides bonded by an iron group metal with hard layers of carbides or nitrides.
Furthermore, it is well known that alumina-coated tools using Al.sub.2 O.sub.3 as a part of the coating in the above described hard alloys have a higher cutting property due to the wear resistance of Al.sub.2 O.sub.3 as a ceramic as compared with the above described coated tools.
As to this Al.sub.2 O.sub.3 layer, .alpha.-type crystal or .kappa.-type crystal Al.sub.2 O.sub.3 has hitherto been proposed and any of the Al.sub.2 O.sub.3 crystal coating can be formed by the chemical vapor deposition method (CVD method). As well known in the art, .alpha.-type Al.sub.2 O.sub.3 is a high temperature stable phase of above 1000.degree. C. and .kappa.-type Al.sub.2 O.sub.3 is formed below this temperature. In view of the relationship with reaction rate, the stable zone of .kappa.-Al.sub.2 O.sub.3 is considered to be in the range of 800.degree. to 1000.degree. C.
Since the strength of ceramics is generally determined in inverse proportion to the grain size of crystal grains thereof, it is thought important to contain smaller crystal grains. In order to reduce the crystal grains, not only formation at a low temperature but also the surface smoothness of a substrate employed are important.